Anode active material for lithium secondary battery and preparation thereof

ABSTRACT

The present invention relates to an anode active material for a lithium secondary battery, comprising a carbon material, and a coating layer formed on the surface of particles of the carbon material and having a plurality of Sn-based domains having an average diameter of 1 μm or less. The inventive anode active material having a Sn-based domains coating layer on the surface of a carbon material can surprisingly prevent stress due to volume expansion which generates by an alloy of Sn and lithium. Also, the inventive method for preparing an anode active material can easily control the thickness of the coating layer.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No. 14/041,280filed on Sep. 30, 2013, which is a continuation of InternationalApplication No. PCT/KR2012/002581 filed on Apr. 5, 2012, which claimspriority under 35 USC 119(a) to Korean Patent Application Nos.10-2011-0031297 and 10-2012-0035464 filed in the Republic of Korea onApr. 5, 2011 and Apr. 5, 2012, respectively, the entire contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an anode active material for a lithiumsecondary battery, and more particularly to an anode active material fora lithium secondary battery, which has a Sn coating layer on the surfaceof a carbon material.

BACKGROUND ART

Different types of electrolytes are being used for recently widely usedelectrochemical devices, for example, lithium secondary batteries,electrolytic condensers, electric double-layer capacitors andelectrochromic display devices, as well as dye-sensitized solar cells ofwhich various studies are being undertaken for future commercialization,and so the importance of electrolytes is increasing day by day.

In particular, lithium secondary batteries are attracting the mostattention due to their high energy density and long cycle life.Generally, a lithium secondary battery includes an anode made of carbonmaterial or lithium metal alloy, a cathode made of lithium metal oxide,and an electrolyte obtained by dissolving a lithium salt in an organicsolvent.

Initially, lithium metal was used as an anode active material for ananode of a lithium secondary battery. However, because lithium has lowreversibility and low safety, currently carbon material is mainly usedas an anode active material of a lithium secondary battery. The carbonmaterial has low capacity compared with lithium, but is advantageous inthat it has a small change in volume, excellent reversibility, and lowprice.

As the use of lithium secondary batteries are expanding, the demand forhigh-capacity lithium secondary batteries are also increasing more andmore. Accordingly, there is a demand for high-capacity anode activematerials that may substitute the carbon material having low capacity.In order to meet the demand, attempts were made to use metals as ananode active material, for example, Si, Sn, and the like, that have ahigher charge/discharge capacity than the carbon material and that allowelectrochemical alloying with lithium.

However, this metal-based anode active material has a great change involume during charging/discharging, which may cause cracks to an activematerial layer. Secondary batteries using this metal-based anode activematerial may suddenly deteriorate in capacity and reduce in cycle lifeover repeated cycles of charging/discharging, and thus, are not suitablefor commercial use.

To solve this problem, attempts have been made to use an alloy of Si andother metal or an alloy of Sn and other metal as an anode activematerial. The use of such an alloy contributes to the improvement ofcycle life characteristics and prevention of volume expansion to someextent when compared with the use of metal alone as an anode activematerial, but the volume expansion generated during alloying withlithium still causes stress, thereby leading to an insufficientcommercial use of secondary batteries.

DISCLOSURE Technical Problem

Therefore, it is an object of the present invention to provide aSn-comprised anode active material for a lithium secondary battery,which can effectively prevent stress due to volume expansion generatedfrom an alloy of Sn and lithium.

Technical Solution

In order to achieve the object, the present invention provides an anodeactive material for a lithium secondary battery, comprising a carbonmaterial, and a coating layer formed on the surface of particles of thecarbon material and having a plurality of Sn-based domains having anaverage diameter of 1 μm or less.

The Sn-based domains may comprise Sn, a Sn alloy or a mixture thereof.The examples of the Sn alloy may be an alloy of Sn and a metal selectedfrom the group consisting of Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn,Ga, Ge, Zr, Nb, Pd, Ag, Cd, In, Sb, Pt, Au, Hg, Pb and Bi.

Preferably, the coating layer has a density of 2.2 to 5.9 g. cm⁻³.

The carbon material which may be used in the present invention includesnatural graphite, artificial graphite, mesocarbon microbeads (MCMB),carbon fibers, carbon black and a mixture thereof. Also, it is preferredthat the carbon material has a specific surface area of 10 m²/g or lessand an average particle size of 5 to 100 μm.

The anode active material of the present invention may be used forpreparing an anode of a lithium secondary battery, comprising a currentcollector and an anode active material layer formed on at least onesurface of the current collector. Such an anode may be used forpreparing a lithium secondary battery including a cathode, an anode, anda separator interposed between the cathode and the anode.

Furthermore, the present invention provides a method for preparing theanode active material for a lithium secondary battery, which comprisesdisposing a main electrode and a counter electrode in an electroplatingbath and filling the electroplating bath with an electroplatingelectrolyte; and dispersing a carbon material in the electroplatingelectrolyte and then applying an electric current to form a coatinglayer having a plurality of Sn-based domains on the surface of particlesof the carbon material.

The electroplating electrolyte which may be used in the presentinvention includes an acid-based Ni—Sn, Sn—Zn, Sn—Co or Sn—Pb platingsolution; a sulfate-based Sn plating solution; a hydrochloride-based Snplating solution; sulfonate-based Sn, Sn—Pb or Sn—Bi plating solution; acyanide-based Sn—Cu or Sn—Ag plating solution; or a pyrophosphate-basedSn, Sn—Cu, Sn—Pb or Sn—Zn plating solution.

Advantageous Effects

In accordance with the present invention, the inventive anode activematerial having Sn-based domains coating layer on the surface of acarbon material comprises Sn in the form of a composite of Sn and thecarbon material, and thus is used in a lithium secondary battery toalleviate stress due to volume expansion generated by an alloy of Sn andlithium.

Also, the inventive method for preparing an anode active material caneasily control the thickness of the coating layer formed on the surfaceof the carbon material by controlling a density of an applied electriccurrent and time. From the inventive method, a semibath-type reactor canalso be designed and an electrolyte used for electroplating can berecovered.

DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate preferred embodiments of thepresent invention and, together with the foregoing disclosure, serve toprovide further understanding of the technical spirit of the presentinvention. However, the present invention is not to be construed asbeing limited to the drawings.

FIG. 1 shows a method for preparing an anode active material inaccordance with one embodiment of the present invention.

FIG. 2 is a scanning electron microscope (SEM) photograph of the surfaceof spherical natural graphite particles having no coating of a Ni—Snalloy.

FIG. 3 is a SEM photograph of the surface of spherical natural graphiteparticles coated with a Ni—Sn alloy, which is an anode active materialprepared in Example 1.

FIG. 4 is a graph showing results evaluating the life characteristics ofbatteries prepared in Example 2 and Comparative Example 1.

BEST MODE

Hereinafter, preferred embodiments of the present invention will bedescribed in detail. Prior to the description, it should be understoodthat the terms used in the specification and the appended claims shouldnot be construed as limited to general and dictionary meanings, butinterpreted based on the meanings and concepts corresponding totechnical aspects of the present invention on the basis of the principlethat the inventor is allowed to define terms appropriately for the bestexplanation.

FIG. 1 shows a method for preparing an anode active material inaccordance with one embodiment of the present invention. However, theconfigurations illustrated in the embodiments and the drawings are justpreferable examples for the purpose of illustrations only, not intendedto limit the scope of the disclosure, so it should be understood thatother equivalents and modifications could be made thereto withoutdeparting from the spirit and scope of the disclosure.

An anode active material for a lithium secondary battery according tothe present invention comprises a carbon material, and a coating layerformed on the surface of particles of the carbon material and having aplurality of Sn-based domains having an average diameter of 1 μm orless.

The anode active material functions to absorb lithium ions contained ina cathode active material during charging and then generate electricenergy along the concentration gradient of lithium ions during thedeintercalation of the absorbed lithium ions. If the anode activematerial is Sn having high charging/discharging capacity, Sn forms analloy with lithium ions during the absorption of lithium ions to causevolume expansion.

In order to solve such a volume expansion problem due to Sn, theinventors have introduced a coating layer comprising Sn-based materialson the surface of a carbon material.

The coating layer comprising Sn-based materials according to the presentinvention is characterized by a plurality of Sn-based domains having anaverage diameter of 1 μm or less. Here, the domains have boundaries, andeach of the domains means the assembly of particles which can beseparated by at least one domain boundary. The Sn-based domains of thepresent invention refer to Sn or a Sn-based alloy. The Sn-based alloymay be an alloy of Sn and a metal selected from the group consisting ofMg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Pd, Ag, Cd,In, Sb, Pt, Au, Hg, Pb and Bi.

The Sn-based domains are formed on the surface of the carbon material tomake a coating layer. The Sn-based domains formed on the surface of thecarbon material may have a hemisphere shape or a porous structureobtained from the agglomeration of several hemispheres. The coatinglayer of the Sn-based domains is uniformly formed on the whole surfaceof the carbon material. Preferably, the Sn-based domains have an averagediameter of 1 μm or less so as to minimize mechanical stress due tocontraction or expansion which is generated in the formation of an alloywith lithium during charging and discharging. That is, when the domainsof Sn or Sn-based alloy components have a smaller size, the anode activematerial can be recovered to its initial size even though thecontraction and expansion is generated during charging and discharging.

The lower limit of an average diameter of Sn-based domains is notparticularly limited. However, Sn-based domains having an averagediameter less than 0.01 μm may be difficult to form physically and thusmay have an average diameter of 0.01 μm or higher.

Also, if the coating layer of Sn or Sn-based alloy having a domain formor a porous structure according to the connection of domains has athickness of several to tens of μm, the coating layer deposited on thesurface of the carbon-based anode active material may be delaminated orbroken during alloying (intercalation) with lithium to deteriorate lifecharacteristics.

Accordingly, the coating layer has a density of 2.2 (corresponding to aporosity rate of 80%) to 5.9 g. cm⁻³ (corresponding to a porosity rateof 20%), preferably 2.3 to 3.2 g·cm^(—3), more preferably 3.3 to 4.5g·cm⁻³, and such a density is higher than that of a Sn-based coatinglayer obtained by an electroless (autocatalytic) plating method whichreduces metallic ions in the presence of a reducing agent in an aqueousmetal salt solution to deposit metals on the surface of a material to betreated without the external provision of electric energy. The Sn-baseddomains are uniformly formed on the surface of the carbon material.Thus, the uniform Sn-based domains coating having a high density canprovide superior battery characteristics to an anode active materialaccording to one embodiment of the present invention.

The carbon material used in the present invention may include any carbonmaterial which is known as an anode active material in the art, withoutparticular limitation, for example, natural graphite, artificialgraphite, mesocarbon microbeads (MCMB), carbon fiber and carbon black.Such a carbon material may be used alone or in a mixture of at least twomaterials, but is not limited thereto.

In addition, it is preferred that the carbon material has a specificsurface area of 10 m²/g or less. If the carbon material has a specificsurface area larger than 10 m²/g, the initial efficiency of the anodemay decrease. In the present invention, the lower limit of the specificsurface area for the carbon material is not particularly limited. Apreferred lower limit of the specific surface area may be 2 m²/g, whichis just for illustration purposes and the present invention is notlimited thereto.

Also, the carbon material may have an average particle size of 5 to 100μm, preferably 5 to 40 on. If the carbon material has an averageparticle size less than 5 μm, the initial efficiency of the anode maydecrease due to the fine powders of the carbon material. If the carbonmaterial has an average particle size larger than 100 μm, theprocessibility for coating an anode slurry may decrease and scratches onan electrode may increase.

The anode active material of the present invention may be prepared by amethod as follows.

First, a main electrode and a counter electrode are disposed in anelectroplating bath and the electroplating bath is filled with anelectroplating electrolyte.

More specifically, as shown in FIG. 1, a Sn-based counter electrode forproviding Sn ions and a main electrode to which Sn is deposited aredisposed in an electroplating bath, and then the electroplating bath isfilled with an electroplating electrolyte. The electroplatingelectrolyte which may be used in the present invention includes, but isnot limited to, an acidic-based Ni—Sn, Sn—Zn, Sn—Co or Sn—Pb platingsolution; a sulfuric acid-based Sn plating solution; hydrochloricacid-based Sn plating solution; sulfonic acid-based Sn, Sn—Pb or Sn—Biplating solution; cyanide-based Sn—Cu or Sn—Ag plating solution; orpyrophosphate-based Sn, Sn—Cu, Sn—Pb or Sn—Zn plating solution. TheSn-based counter electrode for providing Sn ions may be a Sn-basedelectrode or an electrode made of a Sn-based alloy. The main electrodefor the deposition of Sn-based domains may be an electrode made of SuSor Ni, but its type is not particularly limited.

Next, a carbon material is dispersed in the electroplating electrolyteand then applied to an electric current to form a coating layer having aplurality of Sn-based domains on the surface of particles of the carbonmaterial.

More specifically, as shown in FIG. 1, the bath is provided with astirrer to uniformly disperse the carbon material in the electrolyte.The electric current used for electroplating, which flows through thecounter electrode and the main electrode, may be 0.1 to 10 A, and may beused as a pulse current by controlling the time difference and currentvalues of an applied electric current. Preferably, electrodeposition maybe conducted for 10 minutes to 5 hours. As shown in FIG. 1, Sn²⁺ ionsaccept electrons from the main electrode to be deposited on the surfaceof the carbon material.

Such dispersive electrodeposition may be conducted by using asemibath-type reactor which can calculate the maximum value oftheoretical quantities of electrodeposition for Sn or a metal such asNi, Cu, Co, Zn and Pb, used in the formation of a Sn-alloy, in a platingsolution and can easily recover an active material having Sn and aSn-based alloy plated on the surface of the carbon material and theplating solution after dispersive electrodeposition. The recoveredplating solution may be reused.

The anode active material thus prepared according to the presentinvention may be used to prepare an anode by a conventional method knownin the art. Also, a cathode used in the present invention may beprepared by a conventional method known in the art, similar to theanode. For example, the electrode active material of the presentinvention is mixed with a binder, a solvent, and optionally a conductingmaterial and a dispersing agent to produce a slurry, which is applied toa current collector, followed by compression molding, to prepare anelectrode.

The binder which may be used in the present invention includes variouskinds of binder polymers, for example, styrene-butadiene rubber (SBR),vinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, andthe like.

The cathode active material which may be used in the present inventionpreferably includes a lithium-containing transition metal oxide, forexample, any one selected from the group consisting of Li_(x)CoO₂(0.5<x<1.3), Li_(x)NiO₂ (0.5<x<1.3), Li_(x)MnO₂ (0.5<x<1.3), Li_(x)Mn₂O₄(0.5<x<1.3), Li_(x)(Ni_(a)Co_(b)Mn_(c))O₂ (0.5<x<1.3, 0<a<1, 0<b<1,0<c<1, a+b+c=1), Li_(x)Ni_(1-y)Co_(y)O₂ (0.5<x<1.3, 0<y<1),Li_(x)Co_(1-y)Mn_(y)O₂ (0.5<x<1.3, 0≦y<1), Li_(x)Ni_(1-y)Mn_(y)O₂(0.5<x<1.3, O≦y<1), Li_(x)(Ni_(a)Co_(b)Mn_(c))O₄ (0.5<x<1.3, 0<a<2,0<b<2, 0<c<2, a+b+c=2), Li_(x)Mn_(2-z)Ni_(z)O₄ (0.5<x<1.3, 0<z<2),Li_(x)Mn_(2-z)Co_(z)O₄ (0.5<x<1.3, 0<z<2), Li_(x)CoPO₄ (0.5<x<1.3),Li_(x)FePO₄ (0.5<x<1.3), and a mixture thereof. The lithium-containingtransition metal oxide may be coated with a metal such as Al or a metaloxide. In addition, lithium-containing transition metal sulfide,selenide, or halide may also be used.

After the electrode is made, a conventional lithium secondary batteryincluding the cathode, the anode, a separator interposed between thecathode and the anode, and an electrolyte may be prepared.

In the present invention, a lithium salt used as a solute of theelectrolyte is not particularly limited if it is conventionally used inan electrolyte for a lithium secondary battery. For example, an anion ofthe lithium salt may include any one selected from the group consistingof F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻,(CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻,(CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃ SO₂)₂CH⁻, (SF₅)₃C⁻,(CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ², SCN⁻ and(CF₃CF₂SO₂)₂N⁻.

In the electrolyte used in the present invention, an organic solventcontained in the electrolyte is not particularly limited if it isconventionally used in an electrolyte for a lithium secondary battery.For example, the organic solvent may include propylene carbonate (PC),ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate(DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropylcarbonate, dimethylsulfoxide, acetonitrile, dimethoxyethane,diethoxyethane, vinylene carbonate, sulforane, γ-buryrolactone,propylene sulfite, and tetrahydrofuran and or a mixture thereof. Inparticular, among the above carbonate-based organic solvents, ethylenecarbonate and propylene carbonate that are cyclic carbonates arepreferred, since they have high viscosity and consequently a highdielectric constant to easily dissociate the lithium salt in theelectrolyte. More preferably, such a cyclic carbonate is used in amixture with a linear carbonate having a low viscosity and a lowdielectric constant, such as dimethyl carbonate and diethyl carbonate ina suitable ratio, to provide an electrolyte having a high electricconductivity.

Optionally, the electrolyte stored according to the present inventionmay further include an additive such as an overcharge inhibitor which isconventionally used in an electrolyte.

Also, the separator which may be used in the present invention includes,but is not limited to, a single-layered or multi-layered porous polymerfilm conventionally used as a separator, and a porous non-woven fabricconventionally used as a separator, and the like. The porous polymerfilm may be made of polyolefin-based polymer, for example, ethylenehomopolymer, propylene homopolymer, ethylene/butene copolymer,ethylene/hexene copolymer, and ethylene/methacrylate copolymer, and theporous non-woven fabric may be made of, for example, high-melting glassfibers, polyethylene terephthalate fibers, and the like. However, thepresent invention is not limited thereto.

A battery casing used in the present invention may be any oneconventionally used in the art, and the appearance of the battery casingis not limited to a specific shape based on the purpose of use of thebattery. For example, the battery casing may have a cylindrical shape, aprismatic shape, a pouch shape, or a coin shape.

Hereinafter, the present invention will be described in detail throughspecific examples. However, the description proposed herein is just apreferable example for the purpose of illustrations only, not intendedto limit the scope of the invention, so it should be understood that theexamples are provided for a more definite explanation to an ordinaryperson skilled in the art.

EXAMPLES Example 1: Preparation of Anode Active Material

In order to coat Ni—Sn on a carbon-based active material, anelectroplating solution (Ni—Sn electrolyte) was prepared. Specifically,0.075 M nichel chloride hexahydrate (NiCl₂·6H₂O), 0.175 M tin chloridedehydrate (SnCl₂·2H₂O) and 0.125 M glycine (NH₂CH₂COOH) were added to0.5 M potassium pyrophosphate (K₂P₂O₇) aqueous solution, to which 26%ammonia solution (NH₄OH) was added to control pH of the solution.

In 177 g of Ni—Sn plating solution thus prepared, 135 g of sphericalnatural graphite was dispersed to obtain an electroplating electrolyte(pH 8.02) having a solid content of 43.3%.

As shown in FIG. 1, a Sn-plate (counter electrode) and a SuS mesh (mainelectrode) were disposed in 100 ml beaker containing the electroplatingelectrolyte prepared above, which was used as an electroplatingequipment, followed by dispersive Ni—Sn electroplating for 1 hour in thecondition of an electric current of 1.0 A and a stiffing speed of 800 to900 rpm.

The resultant obtained from electroplating was washed with distilledwater at a pH range of 7.0 to 7.51, and dried at 130° C. for 24 hours,to obtain natural graphite particles coated with a Ni—Sn alloy (yield:about 90% or more) as an anode active material.

Example 2: Preparation of Lithium Secondary Battery

The anode active material prepared in Example 1, SBR (styrene-butadienerubber) as a binder, CMC (carboxy methyl cellulose) as a thickener andacetylene black as a conducting material were mixed at a ratio of95:2:2:1 (wt %), and added to water (H₂O) as a solvent to obtain auniform anode slurry. The anode slurry obtained was coated on onesurface of a copper current collector at a thickness of 65 μm, dried andcompressed, followed by punching to the desired size, to obtain ananode.

Ethylene carbonate (EC) and diethyl carbonate (DEC) as a solvent weremixed at a ratio of 30:70 (vol %), and LiPF₆ was added thereto, toobtain 1M LiPF₆ non-aqueous electrolyte.

Also, lithium metal foil was used as a counter electrode, i.e., cathode,and then a polyolefin separator was interposed between both electrodesand the electrolyte obtained was introduced, to obtain a coin-typelithium secondary battery.

Comparative Example 1: Preparation of Lithium Secondary Battery

The procedure of Example 2 was repeated except that spherical naturalgraphite particles having no coating of a Ni—Sn alloy were used, toobtain a coin-shaped lithium secondary battery.

Experimental Example 1: Evaluation for the Surface of Anode ActiveMaterial

The anode active material prepared in Example 1, i.e., the naturalgraphite particles coated with a Ni—Sn alloy on the surface thereof, andthe spherical natural graphite particles having no coating of a Ni—Snalloy were analyzed for their surface by means of scanning electronmicroscope (SEM). The results are shown in FIGS. 2 and 3.

Experimental Example 2: Evaluation for the Cycle Characteristics ofCoin-type Battery using Anode Active Material

The batteries prepared in Example 2 and Comparative Example 1 wereevaluated for charge/discharge characteristics.

Specifically, battery charging was made with a current density of 0.1Cup to 5 mV at constant current (CC)—constant voltage (CV) mode, kept at5 mV at CC mode, and completed when the current density reached 0.005C,and battery discharging was made with a current density of 0.1C up to 1Vat CC mode. The charging/discharging was repeated 50 times under thesame conditions.

The results of the measured charge/discharge characteristics are shownin FIG. 4.

As can be seen in FIG. 4, the battery of Example 2 using naturalgraphite particles coated with a Ni—Sn alloy as an anode active materialrepresents an increased capacity and equivalent cycle characteristicsdue to the Ni—Sn coating, compared to the battery of Comparative Example1 using simple natural graphite particles as an anode active material,and also the battery of Example 2 represents substantially equivalentcoulombic efficiency in the presence of the Ni—Sn alloy coating,compared to the battery of Comparative Example 1.

What is claimed is:
 1. A method for preparing an anode active materialfor a lithium secondary battery, the method comprising: disposing a mainelectrode and a counter electrode in an electroplating bath and fillingthe electroplating bath with an electroplating electrolyte; anddispersing a carbon material in the electroplating electrolyte and thenapplying an electric current to form a coating layer having a pluralityof Sn-based domains having an average diameter of 1 μm or less on thesurface of particles of the carbon material.
 2. The method according toclaim 1, wherein the electroplating electrolyte is an acid-based Ni—Sn,Sn—Zn, Sn—Co or Sn—Pb plating solution; a sulfate-based Sn platingsolution; hydrochloride-based Sn plating solution; a sulfonate-based Sn,Sn—Pb or Sn—Bi plating solution; a cyanide-based Sn—Cu or Sn—Ag platingsolution; or a pyrophosphate-based Sn, Sn—Cu, Sn—Pb or Sn—Zn platingsolution.